You learned how freshwater stickleback populations have lost their body armor through positive selection of a low-armor Eda allele, which is found at low frequency in marine populations. Draw (a) a phylogenetic tree that would support the hypothesis that the low-armor phenotype arose once and then spread throughout freshwater populations and (b) a phylogenetic tree that would support the hypothesis that the phenotype arose independently multiple times in freshwater populations.

(a) The phylogenetic tree supporting the hypothesis that the low-armor phenotype arose once and then spread throughout freshwater populations would show a single branch leading to all freshwater populations with the low-armor phenotype. This would indicate that the low-armor Eda allele originated in a common ancestor of all freshwater populations and then spread through those populations.

(b) The phylogenetic tree supporting the hypothesis that the phenotype arose independently multiple times in freshwater populations would show multiple branches leading to different freshwater populations with the low-armor phenotype. This would indicate that the low-armor Eda allele arose independently in different lineages of freshwater populations, rather than originating in a common ancestor.

In conclusion, the phylogenetic tree can provide evidence for the origin and spread of the low-armor phenotype in freshwater stickleback populations, supporting different hypotheses about the evolutionary history of this trait.